Prop slip improvement as speed increases

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,286
Ok...so it?s late over the pond here in the UK and I?ve had a few beers. Added to this, I?d imagine there is a simple explanation for my question and I realise I could probably find the answer in a more sober state, but hopefully someone can answer this quickly...
At cruising speeds I have a prop slip of about 5 or 6, but at 55-60+mph it reduces to about 2 or 3. Someone please explain this.
One side of my brain is telling me that the boat is more efficient when running high up and next to no hull in the water, but the other half is telling me that some of the prop is breaking the surface. Even if it wasn?t, why would it change, assuming it is getting an uninterrupted flow of smooth water ?
Two known and proven figures (from screen shots and pictures I have are as follows;
Drive=1.47
Prop=19?
Rpm=2380
Gps speed=27.6 mph

Drive and prop as above
Rpm=4680
GPs speed=55mph

Thanks.
 

tpenfield

Moderator
Staff member
Joined
Jul 18, 2011
Messages
17,620
Have another beer :D

Essentially it has to do with the boat being more out of the water at higher speeds, plus a few things about the hydrodynamics of the prop and prop wash. The prop should fully remain in the water as long as you are trimmed OK.

Your prop slippage seems way low . . . just wondering if there is something amiss with your calculations :noidea:
 

porscheguy

Chief Petty Officer
Joined
Jan 17, 2013
Messages
441
A few days ago someone said that prop efficiency increases as pitch goes up. Thats only partially true. The real truth is the higher pitch is only more efficient if both the high and low pitch props are rotating at the same speed. And even then, this is still theoretical. And as stated already, youre getting into hydrodynamics and whatnot.
 
Last edited:

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,286
No Title

Here is a picture and screen shot from the smart nav. Both gps speeds.
The low slip is perhaps because of the amount of money I?ve soent on Props...haha.
Also probably because the boat runs with almost no hull in the water when trimmed up and conditions allow. It?s a well set up wee boat these days....still want an enertia propeller though ! When will it end ?
 

Attachments

  • photo276284.PNG
    photo276284.PNG
    1.4 MB · Views: 0
  • photo278796.PNG
    photo278796.PNG
    99.9 KB · Views: 0

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,286
Oh and thanks for the responses....just about sober now šŸ˜‚
 

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,286
Just thinking about it now...that tachometer in the picture when cruising, always reads a bit high compared to true RPMs. This may explain the difference ? The screen shot shows true RPMs.
Too much beer !
 

porscheguy

Chief Petty Officer
Joined
Jan 17, 2013
Messages
441
What kind of boat, engine, and props do you have? Your 2-3% slip seems very optimistic. Thats usually into race boat territory.

Its also worth mentioning that the trend of recreational boats from the late 80s through the early 00s was high lift, low deadrise, high speed, and good fuel economy. This made a lot boats faster and better in acceleration/planing, but they ride lousy in the slightest chop. So the bulls are very efficient and low drag which is why its so easy to get those low slip numbers. But 2-3% would make me question the tach. If its correct thats great. But I thought you had to have lab finished props to get those results.
 
Last edited:

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,286
It?s a bit of a special campion Chase Performance Edition hull.
V8 mpi with cam and exhaust
Currently running a ballistic prop
The tachometer figure is deadly accurate. Taken from the smart craft.
All the best
 

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,286
I?ll need to wait until next spring to find out what the slip is wot. That will be interesting.
The figure shown is 4600 and something rpm, but she will rev to around 5100 if conditions allow. Round about the Rev limiter for that engine. Max is supposed to be 5000
 

Stumpalump

Chief Petty Officer
Joined
Dec 5, 2013
Messages
413
I'm starting to believe that prop slip numbers are nothing more than an indication of how well the tach is calibrated.
 

QBhoy

Fleet Admiral
Joined
Mar 10, 2016
Messages
8,286
I'm starting to believe that prop slip numbers are nothing more than an indication of how well the tach is calibrated.

I?m certain you are right, but one of the figures is derived from the actual engine management smart craft, so should be pin point accurate.
 

porscheguy

Chief Petty Officer
Joined
Jan 17, 2013
Messages
441
Prop slip is the essentially the difference between the theoretical performance potential, and whats actually happening in the water. The theoretical potential is zero slip. Its kind of like how your prop pitch is theoretical and based on how far the prop will move if screwed into a solid substance. The slip is the difference between that prop being screwed into a solid versus how much it moves through the water.

The number itself doesnt necessarily mean much. Its one piece of information used to help paint a bigger picture.
 
Last edited:
Top